The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described in e.g. U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde--The Focal Press--London and New York (1972).
In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver halide solvent into soluble silver complex compounds which are allowed to diffuse into an image-receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values ("DTR-image") with respect to the black silver image obtained in the exposed areas of the photographic material.
A DTR-image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellent ink-receptive areas on a water-receptive ink-repellent background. For example, typical lithographic printing plates are disclosed in e.g. EP-A-423399 and EP-A-410500.
The DTR-image can be formed in the image-receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion material (a so-called two-sheet DTR element) or in the image-receiving layer of a so-called single-support-element, also called mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image-receiving layer in waterpermeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method.
As for other printing plates it is required that the printing plates obtained according to the DTR-process have a good ink acceptance in the printing areas and no ink acceptance in the non-printing areas (no staining). It is furthermore desirable that the number of copies that have to be disposed of because of ink acceptance in the non-printing areas (so called toning) during start-up of the printing process is limited.
It will be readily understood by those skilled in the art that any measure which would render the non-printing background areas more hydrophilic would be beneficial in avoiding the problem of toning. While the surface of the printing areas consists of hydrophobized DTR-silver the surface of the non-printing background areas is a gelatin surface. So it can be expected that measures rendering the gelatin surface more hydrophilic would be advantageous in avoiding toning. In this context the concept of a modified gelatin comes to aid.
As generally known to those skilled in the art gelatin is prepared from collagen. Details on the preparation of gelatin are described in e.g. "the Science and Technology of Gelatin", A. G. Ward and A. Courts, Academic Press 1977, p. 295. Gelatin consists of a three-dimensional network of polypeptide chains. Each polypeptide chain is built-up by repeating units of about twenty different amino acids linked together by peptide bonds. The dicarboxylic amino acids, i.c. aspartic acid and glutamic acid, provide the free (unbonded) carboxyl groups in the polypeptide chain, while the free amino groups are provided by amino acids, containing more than one amino group, e.g. lysine and arginine. Free carboxylic groups and free amino groups can act as so-called functional groups in several chemical reactions, e.g. modification reactions and hardening reactions. The ratio of free carboxylic and free amino groups determines the so-called isoelectric point, the pH at which the gelatin molecule is electrically neutral.
Scientific and patent literature is replete with references concerning gelatin modifications chemically applied on the free primary amino functions. For instance, different types af acylated gelatins are disclosed in U.S. Pat. Nos. 2,525,753, 2,827,419, 3,486,896 and 3,763,138. Phtaloyl gelatins are described in U.S. Pat. No. 2,725,293 and BE 840,437. Reaction of gelatin with compounds containing active halogen atoms are disclosed in BE 614,426 and BE 1,005,787.
On the other hand disclosures concerning modification on the free carboxyl group are scarce. However such a modification type theoretically would offer several benefits: there are about three times more free carboxyl groups than free primary amino groups offering the possibility of a substantially higher degree of modification, while the latter would remain available for other reactions, e.g. hardening. In U.S. Pat. No. 4,238,480 different reagents, a.o. ethylenediamine, are used to modify collagen into a substance with a more electropositive surface, which is used as a hemostatic agent. In International Application PCT/U.S. Pat. No. 91/04197, published as WO 91/20014, a gelatin is disclosed which is modified by reaction on part of the free carboxyl groups in the presence of (i) an "amide bond forming agent" and (ii) a well-defined type of diamine, triamine or cyclic diamine, e.g. piperazine. In this way additional end-standing amino functions were introduced in the gelatin molecule, which, moreover, proved to be more reactive to vinylsulphonyl hardeners, a common type of hardeners for gelatin, than the original ones. In this way multilayer photographic elements can be designed which show so-called differential hardness.
European Patent Application, filed 8 Mar., 1993, as application No. 93200655 discloses other types of carboxyl group modified gelatin and their use in photographic elements.
The present invention constitutes a further extension on the teachings of modified gelatins cited above be it that the context and objects are quite different.
It is an object of the present invention to provide new types of modified gelatin which show a more hydrophilic character than conventional unmodified gelatin.
It is a further object of the present invention to provide a photographic element, in particular a photographic DTR element, containing such a new type of modified gelatin.
It is a further object of the present invention to provide a DTR element serving as a planographic printing plate having good printing properties with no staining and less toning during start-up.